The present invention relates to an ultrasonic flow meter having a measurement tube through which gas/liquid flows and having ultrasonic emission/reception transducers, which, for a "W" shaped ultrasonic path with multiple reflections provided within the measurement tube, are located at a given spacing from one another on a common side wall of the measurement tube.
Flow meters operated by ultrasound are known from the prior art and are proposed in PCT application PCT/EP90/02178 filed 13 Dec. 1990 and in the parallel patent applications GR 89 P 2014 DE and GR 89 P 2015 DE. The content of the specifications of these applications is an additional component part of the present invention specification.
The flow meters consist essentially of a tube through which the pertinent medium flows and within the interior of which an acoustic path extends. The influencing of the ultrasound by the flowing medium is the parameter from which, for example, the volume of gas or liquid flowing through the tube cross section per unit time is to be ascertained. Such flow meters are suitable, for example, as gas flow meters such as, for example, as domestic gas meters.
The aforementioned acoustic measurement path is formed between an acoustic emission transducer and an acoustic reception transducer. The emission transducer and reception transducer are fitted in a manner known per se to the side wall of the tube through which the medium flows, and specifically in such a manner that an acoustic beam is passed from the emission transducer into the interior of the tube, with a component oriented parallel to the direction of flow. Oblique injection of the ultrasound is the method in widespread use; in this case, the ultrasound is reflected at the opposite wall of the tube and executes, for example with three successive reflections at the inner wall of the tube, an in total W-shaped path between the emission transducer and the reception transducer.
The evaluation of the influencing of the ultrasound by the flow takes place, as a rule, in such a manner that the measurement path is traversed by the ultrasound alternately in a downstream and upstream direction, and the difference value resulting from the influencing of the flow, for the two ultrasound measurements, is the evaluation signal. Thus, the emission transducer and reception transducer are used interchangeably in terms of operation.
The aforementioned W-shaped path is preferred as against an only V-shaped path, because advantages can be achieved with a predetermined spacing, axial when related to the tube axis, between the emission transducer and the reception transducer. It is also possible to provide an ultrasound path with, instead of three reflections, even more reflections, especially an odd number thereof. In this case, it is essential that the ultrasonic transducers do not directly "see" one another, i.e. that ultrasound does not pass without reflection from the respective emission transducer to the pertinent reception transducer.
If use is made of the W-shaped ultrasound path 21 in an arrangement for example as shown in FIG. 1, in which the lower and upper tube wall in the Figure are designated by 4 and 5 and the transducers are designated by 11 and 12, then a signal transmission unavoidably also takes place via the V-shaped path 22 between the emission transducer and the reception transducer. This is based on the fact that the radiation lobes of the emission transducer and the reception lobe of the reception transducer cannot be directed with an arbitrarily selectable degree of sharpness.
In the two aforementioned older patent applications, proposals have been made for the most extensive possible suppression of a parasitic signal of the V-shaped path as compared with the useful signal of the W-shaped path. On the one hand, this involves the use of a markedly rectangular tube cross section, with a ratio of height H to width B exceeding 2:1 to 15:1, preferably 5:1 to 6:1. In addition, such a rectangular cross section has the advantage of acting in a homogenizing manner on the ultrasonic transirradiation of the respective flow cross section. In the other application, it is proposed, in the case of a measurement tube having a not necessarily rectangular cross section, to arrange the transducers 11 and 12 of FIG. 1 in an appropriately modified manner in a "misdirected position". In FIG. 1, the transducers are disposed with respect to their axial spacing from one another and with respect to the angular orientation of their radiatively emitting surfaces 111 and 112 so that the end sections of the W-shaped path do not coincide with the respective normal to these surfaces 111, 112. The misorientation consists in dimensioning the axial spacing to be greater, and/or arranging the transducers 11, 12 with their surfaces 111, 112 "misoriented" at an angle, i.e. tilted, in comparison with the angular orientation of the transducers of FIG. 1. It is also possible to provide these two measures in combination.